The present invention relates to a measuring probe for the polarographic determination of partial gas pressures, in particular, partial oxygen pressure in aqueous solutions, particularly in biologic mediums, comprising an indicator electrode which, except for a very small effective surface, is insulated all around, a diaphragm protecting the effective surface against poisoning, and, if provided, a reference electrode.
Such measuring probes are used in medical and biological examinations if, for example, the partial oxygen pressure (pO.sub.2) in the blood vessels, in the tissue, or even in individual cells is to be determined. The determination of the pO.sub.2 makes it possible to draw conclusions about the metabolic activities or supply conditions in the live tissue.
The known oxygen measuring probes of this kind always comprise a central needle-shaped indicator electrode of platinum, gold, or the like, which is sealed in a glass insulator. The pointed tip of this needle-shaped indicator electrode lies bare as the effective surface of the indicator electrode and is protected against the organic medium by a plastic diaphragm, to prevent a poisoning of the indicator electrode or invalidation of the result, for example, by protein molecules. In most cases, the known measuring probes also comprise a reference electrode, frequently a silver electrode, which is usually also disposed at the tip of the probe, for example, as a tube fitted thereover.
In this kind of known measuring probes, substantially two basic types may be distinguished, namely, macroprobes, having a relatively thick insulating glass body and intended for measurements in blood vessels, and having a tip diameter of approximately 1 mm, with a central platinum wire having a thickness of some 10.mu., and microprobes, which have a measuring tip so fine that they permit piercing individual cells of the tissue and determining the pO.sub.2 thereof. The measuring electrode of a microprobe comprises a wire which is etched from some 10.mu. down to a tip diameter of below 1.mu., and the glass insulator has a diameter less than 0.1.mu. in the tip zone so that a total diameter of approximately 1.mu. is obtained at the tip of the probe. These microprobes also are coated with a plastic diaphragm.
During the measuring operation, measuring current is drawn from the indicator electrode, causing an afflux of oxygen molecules to be recharged at the effective surface of the measuring electrode. A high consumption of oxygen which, in this process, is unavoidable with measuring probes not covered by a diaphragm, leads to a stir effect and to an oxygen depletion in the area immediately adjacent the tip of the probe. Also, the oxygen consumption is related to the effective surface area and inversely dependent on the thickness of the diaphragm. Consequently, with a given effective surface, the diaphragm is provided as thick as possible. However, this strongly reduces the speed of response to the probe, depending on the solubility and rate of diffusion.
The solution to that dilemna might be a diminution of the effective surface but such a solution, however, meets with considerable difficulties. In microprobes, a diminution of the effective surface to a suitable small value results in too small diameters at the tip of the indicator electrode so that the mechanical stability thereof, which is determined by the platinum wire alone, is endangered. In macroprobes, up to date, thick diaphragms have always been used, and the disadvantage of a slow speed of response has been tolerated.
Known constructions of microprobes have still other drawbacks such as the great variation of the effective surface areas of the individual probes which, in the constructions of the prior art, is inevitable. Also, faultless glass sealings are very difficult to make. In most cases, during the manufacture, fine fissures or stresses occur in the glass insulator, which, later on, lead to cracks. Since every crack produces a new, additional, small effective surface, the parameters of such probes scatter considerably. Further, in such cases, the measuring surface is not confined to the tip but extends, in an uncontrollable manner, over a large surface of the shank adjacent the tip.
In addition, with very thin metal pins, as is the case with microprobes, it passes for a rule that glass sealings can be made only if the glass insulator is thinner than the metal pin. That is why microprobes with a satisfactorily small effective surface have always extremely thin glass insulations, the resistance of which to water, understandably, is unusually low, since they very rapidly become thoroughly hydrated. Thereby, the effective surface is enlarged, in an uncontrollable manner, to a multiple. Moreover, in such a case, as well as in the case of the formation of cracks, it is no longer possible to determine the spatial relationship between the measured value of oxygen and the measuring point.